Cold, long wavelength, neutron sources are useful in a variety of analytical, commercial, and medical applications. Cold neutron sources, frequently referred to as cold neutron (CN) sources, provide neutrons with velocities of the order of 2200 m/s and less, and wavelengths typically in the 0.2-10 nm spectral range. Cold, long wavelength, neutrons, i.e. 0.2-10 nm, are highly penetrating and useful for bulk applications requiring significant depth of neutron exposure. CN radiation is useful in microscopy, although resolution is inferior to electron and x-ray microscopy. The optical cross section for neutrons in neutron analysis is dominated by atomic nucleii. This is in contrast to electron and x-ray beams that are modulated by electron shell structure. Thus neutron beams see different structural landscapes in microscopy and add a new dimension.
Neutron beam facilities with higher beam intensities are coming on line, and promise a variety of important new and valuable applications. Neutron activation analysis, in which a material sample is exposed to a neutron beam and neutron spin properties detected, is a widely used and important technique for determining composition of matter. With highly focused CN beams microscopic samples, and microscopic regions of samples, can be analyzed. Spatial variations in composition over small areas can be resolved.
In semiconductor device manufacturing, CN sources are useful for non-destructive testing of semiconductor crystal structures for defect analysis and impurity profile analysis. Strain distribution in semiconductor crystals can be revealed by CN analysis and is used in the design and production of semiconductor lasers to predict device lifetime. High intensity and highly focused beams improve both spatial resolution and detection limits in these analyses.
CN beams are useful in medicine for abnormal tissue therapy. High flux beams are desired to reduce exposure time, and highly localized beams are beneficial in reducing radiation exposure of adjacent healthy tissue. In these, and other applications, some yet to be fully realized, the utility of the CN tool is usually in direct proportion to the intensity of the beam, and the control of the beam direction, i.e. the ability to focus CN beams. At the present time, both reactor (continuous) and spallation (pulsed) sources of cold neutrons suffer from very low total fluence. This fact severely limits the usefulness of CN apparatus in most applications.
CN beam lensing elements have been sought for some time both to focus the beam and increase the neutron flux density, and to simplify beam handling, i.e. manipulation and steering. Lensing elements can also be important to modify the angular divergence of a neutron beam in two circumstances. The first is in matching the cold neutron source to guide tubes, in which divergence needs to be matched to the critical angle for total internal reflection. The second is in scattering applications where the beam divergence is an important issue. In this case a lens, similar to an infinity corrected optic, can be used to reduce the beam divergence from a pinhole or other source.
Efforts to focus CN beams have met with only mild success. The best results to date have been with lenses and collimators based on reflective optics. It has been known for some time that neutrons will undergo nearly total reflection from a variety of materials. The critical angle however, is typically very high, leading to beam steering devices based on lightguide approaches. A widely used device of this kind is an array of capillary guides, sometimes referred to as a Kumakhov lens, and supermirror coated guide tubes. The capillaries are typically glass or plastic with the interior of the capillary coated with a neutron reflecting material, e.f. nickel. The individual capillaries are arrayed in a parallel bundle, closely packed to capture as much of the source beam as possible. The source then becomes in actuality a multiple beam source. The capillaries are bent inwardly with respect to the axis of the bundle to focus each of the multiple beams to a common focal point. For more details on these systems see e.g., U.S. Pat. No. 5,497,008, issued Mar. 5, 1996; M. A. Kumakhov and V. A. Sharov, "A neutron lens", Nature, Vol. 357, 4 Jun., 1992, pp. 390-393,; H. Chen et. al., "Neutron focusing lens using polycapillary fibers, Appl. Phys. Lett. 64 (16), 18 Apr. 1994, pp. 2068-2070; Q. F. Xiao et al, " Neutron focusing optic for submillimeter materials analysis, Rev. Sci. Instrum. 65 (11), November 1994, pp. 3399-3402.
Neutron optics can also be important to defocus, or magnify, a neutron beam. An example in small neutron scattering is the case when resolution is limited by the fixed (and not optically small) spatial resolution of two-dimensional neutron detectors. A magnifying lens could be used to optimize the spatial variation of the signal in the plane of the detector.
The devices described in the references given above, and other focusing devices based on grazing angle reflection of neutrons, are typically difficult and expensive to make. Moreover, and perhaps more significantly, they are inefficient because a substantial fraction of the already low flux source beam is lost due to the unused space between capillaries. While in an ideal model, with zero wall thickness for the capillaries, and the capillaries arranged in a hexagonal close packed array, the loss due to interstitial space is only 9.3%, when the wall thickness and the thickness of the interior wall reflective coating is considered, with a typical wall thickness equal to at least 20% of the ideal diameter (OD), the interstitial loss approaches 50%. That large loss factor could be eliminated with a refractive optics lens, but to date no such lens exists.